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The new research involving 99 chimpanzees from a wide range of ages showed that genes explained about 50% of the differences seen in their intelligence test scores.

Chimps raised by human caretakers did no better in the tasks than individuals brought up by their chimpanzee mothers.

Study leader Dr William Hopkins, from the Yerkes National Primate Research Center in Atlanta, US, said: ''Chimps offer a really simple way of thinking about how genes might influence intelligence without, in essence, the baggage of these other mechanisms that are confounded with genes in research on human intelligence.

''What specific genes underlie the observed individual differences in cognition is not clear, but pursuing this question may lead to candidate genes that changed in human evolution and allowed for the emergence of some human-specific specialisations in cognition.

''It is also intriguing to consider what changes in cortical organisation might be associated with individual differences in cognition and whether common genes might explain their common variance.''